3D displacement of the middle ear ossicles in the quasi-static pressure regime using new X-ray stereoscopy technique

Vibration Measurements of the Gerbil Eardrum Under Quasi-static Pressure Sweeps

Tympanometry provides an objective measurement of the status of the middle ear. During tympanometry, the ear-canal pressure is varied, while the response of the ear to sound pressure is measured. The effects of the pressure on the mechanics of the middle ear are not well understood. This study is a continuation of our previous work in which the vibration response of the gerbil eardrum was measured in vivo under quasi-static pressure steps. In this study, we delivered a continuous pressure sweep to the middle ear and measured the vibration response at four locations for six gerbils. Vibrations were recorded using a single-point laser Doppler vibrometer and glass-coated reflective beads (diameter ~ 40 µm) at the umbo and on the mid-manubrium, posterior pars tensa and anterior pars tensa.The vibration magnitudes were similar to those in the previous step-wise pressurization experiments. Most gerbils showed repeatability within less than 10 dB for consecutive cycles. As described in the previous study, as the frequency was increased at ambient pressure, the vibration magnitude on the manubrium increased slightly to a broad peak (referred to as R1) and then decreased until a small peak appeared (referred to as R2), followed by multiple peaks and troughs as the magnitude decreased further. The low-frequency vibration magnitude (at 1 kHz) decreased monotonically as the pressure became more negative except for a dip (about 500 Pa wide) that occurred between - 700 and - 1800 Pa. The lowest overall magnitude was recorded in the dip at mid-manubrium. The vibration magnitudes also decreased as the middle-ear pressure was made more positive and were larger than those at negative pressures. R1 was only visible at negative and small positive middle-ear pressures, while R2 was visible for both positive and negative pressures. R2 split into multiple branches after the middle-ear pressure became slightly positive. No magnitude dip was visible for positive middle-ear pressures.The low-frequency vibration magnitudes at negative middle-ear pressures on the pars tensa were higher than those on the manubrium. R1 was not visible for large negative middle-ear pressures on the pars tensa. R2 appeared as a multi-peak feature on the pars tensa as well, and a higher-frequency branch on the posterior pars tensa appeared as a trough on the anterior pars tensa. The magnitude dip was not present on the pars tensa. The largest overall magnitude was recorded at the R2 peak on the posterior pars tensa.The results of this study expand on the findings of the step-wise pressurization experiments and provide further insight into the evolution of the vibration response of the eardrum under quasi-static pressures.

Performance Evaluation of Coupling Variants for an Active Middle Ear Implant Actuator: Output, Conductive Losses, and Stability of Coupling With Ambient Pressure Changes

INTRODUCTION

This study aims to investigate the performance of an active middle ear implant actuator for various coupling configurations. Actuator output and conductive losses were measured, and the stability of coupling was evaluated by challenging the link between actuator and ossicles through pressure events in magnitudes that occur in daily life.

METHODS

Actuator coupling efficiency and the occurrence of conductive losses were measured in 10 temporal bones through laser Doppler vibrometry on the stapes footplate for various coupling types (incus short process with and without laser hole, incus long process, stapes head). To test the stability of coupling, actuator output was measured before and after daily-life pressure events that were simulated; Valsalva maneuvers (500 cycles of -40 to +60 hPa) and jumping into a swimming pool and diving 3 m deep (a step change of 300 hPa).

RESULTS

Actuator output was similarly high for all types of coupling to the incus (short process and long process) and most efficient for coupling to the stapes head. Conductive losses occurred in two temporal bones (TBs) for short process coupling but for seven TBs for coupling to the incus long process. All coupling types were stable and did not lose efficiency after pressure events in the low-frequency range (<1 kHz). Losses in output of 13 to 24 dB were observed in one TB at frequencies from 3 to 6 kHz.

CONCLUSION

Actuator output was similarly high for all types of coupling to the incus but coupling to the incus long process led to a higher occurrence of conductive losses. All three coupling configurations connected the actuator securely to the ossicular chain, under variations of barometric pressure that can be expected in daily life.

Tympanic membrane pressure buffering function at quasi-static and low-frequency pressure variations

Deformation of the tympanic membrane is known to contribute to the pressure regulation processes in the middle ear cleft. In this paper we investigated pressure variations in the rabbit middle ear in response to sinusoidal varying pressures applied to the ear canal, with frequencies ranging from 0.5 Hz to 50 Hz and pressure amplitudes ranging between 0.25 kPa and 1 kPa. The transtympanic pressure difference was found to be smallest in the quasi-static range, and quickly increased as a function of frequency. The response curves showed asymmetry, with larger transtympanic pressures when positive pressures were applied in the ear canal. Normalized transtympanic pressure amplitudes remained fairly constant as a function of input pressure, with values in the range of 60%-70% relative to the applied pressure. The total harmonic distortion of the middle ear pressure signal was calculated and was found to be very small (≤2%) for low-pressure amplitudes and low frequencies. For pressure amplitudes in the order of 0.25 kPa-0.5 kPa, it increased to about 10% at 50 Hz. When a 1 kPa pressure amplitude was applied, variation between animals became large and distortion values up to 30% at 50 Hz were observed. The results showed that pressure buffering due to tympanic membrane displacement was most effective for compensating small transtympanic pressure loads at low frequencies.